In the field of communications transceivers, microprocessors are utilized for controlling the transceiver as well as much of the signal processing. The microprocessor is in turn controlled by a crystal oscillator which provides clock pulses for the operation of the microprocessor. Interference, or self quieting, occurs when harmonics of the clock frequency of the crystal oscillator fall within the passband of the receiver in the transceiver, because the receiver cannot distinguish between a valid carrier and the harmonic.
Self quieting can be eliminated if two separate clock frequencies are provided with harmonics that do not simultaneously fall within the passband of the receiver. The problem is that a large frequency shift, or separation is required between the two frequencies, especially in lowband frequency operation. This large frequency shift is difficult to be implemented using single crystal oscillators. Crystal are generally stiff and can be shifted to a maximum of about 300 parts per million (PPM) or cycles per megahertz of operating frequency without much problem. However, this amount of shift is not sufficient, especially in lowband operation.